Staff members who work for both President Joe Biden and Vice President Kamala Harris had no indication Biden had made his decision to step down from the 2024 race, sources told ABC News.

The administration's staff was not aware, even as his letter was being distributed, sources said.

-ABC News' Aaron Katersky

Transportation Secretary Pete Buttigieg, who ran against Biden in 2020, commended President Biden’s decision to leave the race in a post on X.

"Joe Biden has earned his place among the best and most consequential presidents in American history," he wrote. "I am so proud to serve under his leadership, and thankful for his unwavering focus on what is best for our country."

Former DNC Chair Donna Brazile is backing Vice President Kamala Harris in the 2024 election and predicted she would be the nominee, but left open the possibility of an open convention that would land her the nomination.

"Part A was Biden-Harris. Part B was Harris and her running mate. Part C would likely be an open convention” where Harris would likely prevail, Brazile, an ABC News contributor, told ABC News Live.

Harris has not yet said whether she will be running for president.

"She would be and is the strongest candidate," Brazile said.

After Biden's announcement that he would end his reelection campaign, Michigan Gov. Gretchen Whitmer praised him in a post on X.

"President Biden is a great public servant who knows better than anyone what it takes to defeat Donald Trump," she wrote. "His remarkable work to lower prescription drug costs, fix the damn roads, bring supply chains home, address climate change, and ensure America's global leadership over decades will go down in history."

"My job in this election will remain the same: doing everything I can to elect Democrats and stop Donald Trump, a convicted felon whose agenda of raising families' costs, banning abortion nationwide, and abusing the power of the White House to settle his own scores is completely wrong for Michigan," she wrote.